1. Technical Field
The present invention relates to improvements in contact sensing and measuring probes for measuring apparatus. More particularly, the present invention relates to a probe particularly suited for a coordinate measuring machines in which a portion of the probe deflects (or moves from) its rest position in response to contacting a workpiece surface, then returns to its rest position when contact with the workpiece surface ceases. The present invention is a probe which can be used either as a "digital" probe (providing an on/off signal in response to probe contact) or an "analog" probe (generating a signal the value of which is related to the amount of probe displacement) or as a scanner following the contour of a part, giving dimensional (deviation) information.
2. Background Art
Contact-sensing probes are known in the art in which the probe includes a movable probe stylus mounted in a housing by one or more mechanical springs, and the springs, together with internal structure associated with the stylus and the housing, urge the stylus desired rest position in contact with an electrical circuit. Movement of the stylus away from the rest position breaks the electrical circuit, generating a signal indicative of contact between a part and the stylus. Such probes are shown in U.S. Pat. No. 4,136,458 to Bell et al.; U.S. Pat. Nos. 4,153,998; 4,270,275 and 4,073,314, all to McMurtry, who is also an inventor of probe systems described in UK Patent Specifications No. 1,593,050; 1,573,447 and 1,597,842.
In the probes for coordinate measuring machine applications, it is desirable that the stylus return to its rest position accurately and uniformly so that measurements taken by the measuring machine and probe are uniform.
Probes in which the stylus is spring-biased into its rest position exhibit directional characteristics which have preferred orientations and are not uniform. The spring biased rest positions lead to lobing errors wherein a greater triggering force or displacement is required in one direction than in another direction. Such a preferred direction is especially exhibited in those arrangements where the stylus is suspended within the probe housing by a three-point seat, created by three balls or three rods extending outwardly from the stylus at 120 degrees one from the next and mated within coverging surfaces formed by rods or V-shaped grooves. Such a system is illustrated by various prior art designs including that shown in U.S. Pat. No. 4,270,275.
In such spring-biased probes, the spring provides an initial force to oppose a triggering displacement in response to contact as well as a return force. Ideally, the return force would be high to provide a fast reseating of the probe stylus to insure precise seating, but the use of a strong spring undesirably raises the force necessary to trigger the probe. Thus, the selection of a spring force represents a trade-off between a high recentering force and high triggering force in certain prior art probes when the recentering function and the resistance to triggering is provided by a common element.